1. Field of the Invention
The present invention relates to a charging device which charges the surface of a material, and more particularly to a charging device which includes at least a charging member and a cleaner for cleaning the charging member and which charges an image bearing member such as photoreceptors in an electrophotographic image forming apparatus such as copiers, facsimile machines and printers. In addition, the present invention also relates to an image forming apparatus and a process cartridge using a charging device.
2. Discussion of the Background
Recently, process cartridges including a unit of, for example, a photoreceptor and one or more of image forming devices such as charging devices, developing devices, and cleaning devices are broadly used for electrophotographic image forming apparatus. Even when one or more image forming devices are damaged or the lives thereof expire, the image forming apparatus can be continuously used with a short downtime by replacing the process cartridge with new one. In this case, only the replacing operation is performed, and therefore the operation time of the serviceman is very short. Alternatively, a user may perform the operation without calling a serviceman because the replacing operation is easy.
When process cartridges include a charging member and a cleaning member for cleaning the charging member which are integrated with each other as a unit while the cleaner is brought into pressure-contact with the charger, a problem in that the cleaning member is deformed when the process cartridges are preserved before use. In this case, the deformed cleaning member has poor cleaning ability.
In addition, recently toners having a small particle diameter or polymerization toners which are prepared by a polymerization method are typically used in order to produce high quality images and to save energy in the toner manufacturing process. However, these toners tend to pass through the nip between a cleaning member and a surface of a charging member, namely, the toners have poor cleaning property. Particularly, when such a deformed cleaning member as mentioned above is used, the toner passing problem is caused more frequently.
When the amount of toner particles remaining on the surface of the charging member without being removed increases due to the toner passing problem, a toner layer is formed on the charging member, thereby causing defective charging.
In attempting to solve the problem, published unexamined Japanese patent application No. (hereinafter referred to as JP-A) 07-199603 discloses an image forming apparatus in which pressing members, each including a cleaner, press a charging roller and a transfer roller to a photoreceptor via the respective cleaners, wherein the charging roller and the transfer roller are separated from the photoreceptor or are contacted with the photoreceptor by their own weights when they are not used for forming images. Since the charging roller and the transfer roller are not contacted with or are lightly contacted with the photoreceptor, a problem in that the charging roller and the transfer roller are deformed due to a strong contact pressure, which results in deterioration of image qualities, is not caused.
JP-A 01-207768 discloses an image forming apparatus which uses a contact charging device which is allowed to separate from the photoreceptor by a mechanical method when the apparatus is in a standby state (i.e., when image forming operations are not performed). In addition, JP-A 03-35276 discloses an image forming apparatus which uses a contact transferring device which is allowed to separate from the photoreceptor by a mechanical method when the apparatus is in a standby state.
However, the above-mentioned techniques are used for preventing deformation of the charging devices or the transfer devices after the devices are set in the image forming apparatus, and are not used for preventing deformation of the members when the devices are preserved (i.e., when process cartridges including the devices are not set in the image forming apparatus).
JP-A 09-96945 discloses an image forming apparatus which includes a charging device including a charging roller configured to charge a photoreceptor, a cleaning pad which is supported so as to be contacted with or separated from the charging roller, and a means for moving the cleaning pad such that the portion of the cleaning pad contacted with the charging roller is changed, wherein the photoreceptor drives the contact portion moving means to move without providing a special driving means. By moving the cleaning pad, the contact portion is changed and thereby the above-mentioned deformation problem can be avoided. Although this technique is intended to miniaturize the image forming apparatus by not providing a special driving means, the image forming apparatus has a complicated structure and therefore the image forming apparatus cannot be miniaturized.
Because of these reasons, a need exists for a charging device which can be used for a long period of time without causing the deformation problem even when the charging device is preserved for a long period of time while incorporated in a process cartridge.